Wells drilled to access subsurface formations may be lined with tubular members, typically metal tubular lengths joined together by threaded connectors. In the oil and gas exploration and production industry, an initial section of the well bore is drilled and then lined with a string of tubular members, known as casing, which extends from the end of the bore to the surface of the earth. Cement may then be circulated into the annulus between the casing and the bore wall. The set cement provides support for the bore wall and prevents fluid migration along the annulus. The drilling of the bore is then continued beyond the end of the first casing. A second casing string is then run into the bore. This string also extends from the end of the bore to surface. Again, cement is circulated into the annulus between the casing string and the bore wall. However, the volume of cement is selected to be sufficient only to fill the annulus between the second string and the surrounding unlined bore wall; the annulus between the first and second casing strings is left substantially free of cement.
This process is repeated until the desired depth is reached, and the upper end of the bore is lined by numerous casing strings, the smallest diameter innermost casing extending from surface to the end of the bore.
At some point in the life of the well, for example prior to abandonment or in the course of workover procedures, it may be desired to remove portions of some of the casing strings. Generally, no attempt is made to retrieve the cemented casing sections. Rather, the innermost casing is cut above the cemented section, and the cut section retrieved. However, this may not be straightforward. Solid material may have gathered or settled in the annulus between the cut casing and the surrounding casing during the life of the well. Also, corrosion between the casings may cause adjacent casings to become fixed relative to one another.
Thus, a typical procedure for retrieving casing may involve the following steps:                determining the height of the cement in the annulus between the inner casing section to be cut and retrieved and the larger outer casing;        determining the location of the cut to be made in the inner casing;        setting a packer below the cut location to seal off the bore of the inner casing;        cutting the inner casing;        running a work string into the casing carrying a fishing bottom hole assembly (BHA) comprising a casing spear and a pack-off element;        setting the casing spear and the pack-off element at the upper end of the casing to be retrieved;        applying tension to the cut casing via the work string and casing spear; and        simultaneously applying fluid pressure to the isolated section of the cut casing between the lower packer and the upper pack-off, via the work string, to push or pump the casing free.        
If the cut casing does not come free a second cut may be made in the casing, closer to surface.
In anticipation of difficulties in retrieving a cut casing, the operator may incorporate a jar in the fishing BHA. Alternatively, the present applicant's Agitator (Trade Mark) tool may be incorporated in the BHA. As described in U.S. Pat. No. 7,077,205, the disclosure of which is incorporated herein in its entirety by reference, a flow pulsing tool such as the applicant's Agitator tool may be used in conjunction with an extension and retraction means, such as a shock tool, to vary the tensile load applied to a stuck object, such as a cut casing section. Operation of the applicant's Agitator tool, further details of which are described in U.S. Pat. No. 6,279,670, the disclosure of which is also incorporated herein in its entirety by reference, requires circulation of fluid through the tool. Thus, fluid is pumped down the work string, passes through the Agitator tool, and passes though outlet ports in the fishing BHA and into the annulus above the casing spear.